Separating sand from aquasoil

After attending AGA's 2008 convention i've come home inspired. One thing i really liked is that in Amano's demonstration, he used rocks wrapped in moss to separate the substrate from the decorative sand.

I really want to try something like this. Right now i have my substrate and sand divided by rocks that are halfway in the sand. My main goal though is to do something so that the rocks will disappear, however moss may not work.

Do you guys have any ides on how to make rocks that are separating substrate from sand disappear? The main thing is that my foreground is mostly crypt parva and hairgrass, and having a line of moss will look out of place.